1. Field of the Invention
The present invention relates to a method for manufacturing a cam shaft which is arranged to bulge and deform a tubular shaft member having a hollow portion in the radial direction after fitting thereon cam pieces having shaft holes so as to joint said shaft member with the cam pieces.
2. Description of the Prior Art
Recently, in an automobile engine, the number of cam shafts is in the increasing trend due to adoption of DOHC mechanism, V-shaped arrangement of cylinders, etc., but accompanying this trend, there is such a problem that the valve operating system of the engine becomes large-sized and heavy-weighted. Therefore, methods for the weight reduction of the valve operating system by making the shaft hollow have been proposed. For example, a method for forming a cam shaft which is arranged to bulge and deform a hollow tubular shaft member in the radial direction after fitting thereon cam pieces having shaft holes a little larger than the outer diameter of said shaft member so as to press the outer circumferential surface of the shaft member into contact with the inner circumferential surface of each of said cam pieces, thus to joint said shaft member with said cam pieces has been proposed.
Concretely, there has been proposed a manufacturing method for a cam shaft (the so-called bulge processing) which is arranged to bulge and deform in the radial direction a hollow tubular shaft member after fitting cam pieces thereon by feeding high pressure fluid into the hollow portion thereof, thus to joint the shaft member with the cam pieces (for example, Japanese Patent Application Publication Tokukohei No. 2-62333).
In addition, there has been also proposed a manufacturing method for a cam shaft which is arranged to insert, after fitting cam pieces on a tubular shaft member, a steel ball or an expanding member having a diameter a little larger than the diameter of the hollow portion of said shaft member into the hollow portion of the shaft member, thus to joint the shaft member with the cam pieces (for example, Japanese Utility Model Laid-Open Publication Jitsukaisho No.58-113805).
However, in these conventional manufacturing methods for the cam shaft, there has been such a problem that the bonding force between the shaft member and the cam pieces are comparatively weak and the durability of the cam shaft is low. Furthermore, there has been such a problem that the positioning of the cam pieces is difficult.
The present invention has been developed to solve such conventional problems as described above, and an essential object of the present invention is to provide a cam shaft manufacturing method which is capable of strongly bonding the shaft member and the cam pieces together and easily effecting the positioning of the cam pieces.